The horsemen in pairs may be part of the same army described in Isaiah 21:7, or possibly a smaller group bringing the tidings of victory. Finally, Babylon would fall, her idols shattered.
Although Babylon existed in Isaiah's time, the seat of world power was in Assyria. Only later would Babylon rise to power and become a great oppressor of God's people. This would lead to Judah's exile and the destruction of Jerusalem and the temple. What is announced in this verse is the basic theme of the entire book: God has delivered his people from their enemies. It is a prelude to the messages of consolation we find in Isaiah 40–66.
The army described here is that of the Persian king Cyrus, who conquered Babylon in 539 BC, allowing the faithful Judahites to return to their land.
9 And behold, here come riders, horsemen in pairs!” And he answered, “Fallen, fallen is Babylon; and all the carved images of her gods he has shattered to the ground.”